Many of these are now being listed as Panama Canal, Trans Atlantic and South America cruises.
The repositioning cruises reposition the ship from one season's itinerary to another's. Alaska to the Caribbean, Europe to the Caribbean, etc.
For others reading this thread: It's always a good idea to look at the cruise air price for these cruises. They often involve open jaw (fly into one airport, return from another) or international air. Cruise line air is often less expensive than you can find it on your own. When I book a European cruise or a cruise that has open jaw air I book the cruise line air at the time I put my deposit down. If, at a later time, I find the air at a lower price I can always cancel the air portion of the trip.